Method for decelerating a cooling down of a flow conducting unit, and flow conducting unit

ABSTRACT

A method for making a flow conducting unit cool down more slowly where the flow conducting unit has a holding unit and is penetrated by a heat transfer medium, the temperature of the heat transfer medium is reduced such that a wall of the flow conducting unit cools down, and the holding unit dissipates thermal energy such that the wall cools down more slowly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2016/058639 filed Apr. 19, 2016, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 102015210156.8 filed Jun. 2, 2015. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a method for decelerating a cooling down of a flow conducting unit. The invention also relates to a flow conducting unit.

BACKGROUND OF INVENTION

A large number of technical plants have flow conducting units which are exposed to throughflow by a hot heat transfer medium. If during the course of an operation of such a plant cooling of the heat transfer medium occurs as a result of altered process conditions, the flow conducting units also cool down. In order to decelerate a cooling down of the flow conducting units and thereby to reduce thermal energy losses of the plant, known flow conducting units have up to now comprised thermal insulations with a low thermal conductivity or a low heat transfer coefficient.

Despite such thermal insulations, the flow conducting units release a lot of thermal energy to their environment and cool down in the process so that during renewed heating of the heat transfer medium a lot of thermal energy is released by the heat transfer medium to the flow conducting unit. This counteracts an energy efficient operation of the plant.

SUMMARY OF INVENTION

It is an object of the present invention to specify a method by means of which a cooling down of a flow conducting unit can be decelerated in an efficient manner.

This object is achieved according to the invention by means of a method for decelerating a cooling down of a flow conducting unit, in which the flow conducting unit comprises a heat retention unit and is exposed to throughflow of a heat transfer medium, a temperature of the heat transfer medium is reduced, as a result of which a wall of the flow conducting unit cools down, and the heat retention unit releases thermal energy, as a result of which a cooling down of the wall is decelerated.

The decelerating of a cooling down can in this context be understood as being a deceleration of a temperature decay. That is to say, if a cooling of the flow conducting unit is decelerated, its temperature loss rate or its cooling down rate decreases for logical reasons.

The heat retention unit can release thermal energy indirectly or directly to the wall. This can counteract a cooling down of the wall. As a result, the cooling down of the wall can be decelerated. A plant can thereby be operated in an energy efficient manner.

In one advantageous embodiment of the invention, the heat retention unit comprises a heating device. The heating device can release thermal energy, especially to the wall. Furthermore, the heating device can release the thermal energy indirectly or directly to the wall. In the case of the heating device it can be for example an electric heating device, especially an ohmic heater.

According to a further development of the invention, a temperature of the wall is held at, or above, a predetermined temperature threshold value, especially at, or above, 400° C., with the aid of the heating device.

The heat retention unit advantageously comprises a phase change material. The phase change material can undergo a phase change from a first phase into a second phase. Also, the phase change material can release thermal energy during the phase change. As a result, a cooling down of the wall can be decelerated.

During the cooling down of the heat transfer medium a part of the phase change material or the entire phase change material expediently undergoes a phase change, also referred to as a phase transition.

With the aid of the phase change material, the thermal mass or the thermal inertia of the flow conducting element can be increased. Consequently, a temperature retention can be improved.

In principle, the heat transfer medium can be a vapor or a liquid. The heat transfer medium is advantageously steam.

The thermal energy which is released by the phase change material during the phase change is advantageously released to the wall. The wall can in turn transmit the thermal energy to the heat transfer medium. Moreover, the thermal energy which is released by the phase change material during the phase change can be released to the heat transfer medium. The heat transfer medium can in turn transmit the thermal energy to the wall. As a consequence of the phase change of the phase change material, the cooling down of the heat transfer medium is also expediently decelerated.

In an advantageous embodiment of the invention, the first phase is a liquid phase. The second phase is expediently a solid phase. Furthermore, the entire phase change material or a part the phase change material can solidify during the cooling down of the heat transfer medium. The phase change from the liquid phase into the solid phase for logical reasons takes place within the range of a solidifying temperature of the phase change material. During the solidification, the phase change material expediently releases thermal energy in the form of enthalpy of fusion.

So that the phase change material can undergo a phase change from the first phase into the second phase, it expediently first of all has to be brought into the first phase. The phase change material is advantageously brought from the second phase into the first phase, especially melted, by it being heated.

Alternatively, a different phase change than the phase change from a liquid phase into a solid phase can also be used. Therefore, for example the first phase can be a solid phase. The second phase can also be another solid phase. A phase change from a solid phase into another solid phase can be characterized for example by a change of a crystalline structure of the phase change material. The entire phase change material or a part of the phase change material can also change the crystalline structure during the cooling down of the heat transfer medium. During the change of the crystalline structure, the phase change material expediently releases thermal energy.

The thermal energy which is released or absorbed by the phase change material during the phase change is expediently latent heat. The phase change material can therefore be seen as a heat accumulator, especially as a latent heat accumulator.

It is advantageous if the phase change material has a lower thermal conductivity than the wall. As a result, the phase change material can thermally insulate the wall. However, the thermal conductivity of the phase change material is expediently is at such a level that a sufficiently large heat flux from the phase change material to the wall is ensured.

In an advantageous embodiment of the invention, thermal energy is fed to the phase change material by means of a heating device, especially if a temperature of the phase change material falls short of a predetermined minimum temperature. The last-mentioned heating device can especially be the previously mentioned heating device. The phase change material can also be heated directly or indirectly by means of the heating device. In particular, by means of the heating device the wall can be heated and can transmit the thermal energy to the phase change material.

In an advantageous further development of the invention, the method can be used when operating a power plant, especially a thermal power plant. For example, a cooling down of a flow conducting unit of the power plant can be decelerated according to the method described above. Consequently, a starting process of the power plant can be more efficiently, especially more energy efficiently and time efficiently, designed. For logical reasons, the flow conducting unit of the power plant is the previously mentioned flow conducting unit. The temperature of the heat transfer medium which flows through the flow conducting unit is advantageously reduced as a result of a reduction of output of the power plant.

The output of the power plant can for example be reduced by a fuel feed into a combustion chamber of the power plant being reduced. Furthermore, the temperature of the heat transfer medium can especially be reduced by the power plant being run down.

During the running down of the power plant, the flow conducting unit can cool down. It can also be necessary to heat the flow conducting unit during the (re)starting of the power plant initially above a temperature threshold value before a steam turbine of the power plant can be put into operation and the power plant can feed energy into an electricity supply network.

It is therefore practical to decelerate the cooling down of the flow conducting unit during the running down of the power plant. By means of the method described above, the effect of the flow conducting unit of the power plant having a higher temperature during a (re)starting of the power plant, especially within 24 to 48 hours after the running down of the power plant, can be achieved. As a result, the time until the flow conducting unit, especially its wall, is heated to the temperature threshold value, can be curtailed in turn. Consequently, the duration of the startup process of the power plant can be curtailed.

In particular, as a result of the faster starting the power plant can be operated more efficiently. Moreover, as a result of smaller temperature fluctuations on account of the decelerated cooling, thermomechanical loads upon the flow conducting unit can be reduced. Consequently, the power plant can be operated in a material-conserving manner on account of the reduction of the thermomechanical loads. This leads to the service life of the flow conducting units being able to be increased. As a result, the power plant can be operated in a low-wear and therefore low-maintenance manner. Furthermore, the efficiency of the power plant can be improved by the use of the heat retention unit. Overall, the power plant can therefore be operated more economically.

The invention also relates to a flow conducting unit. So that a cooling down of the flow conducting unit can be decelerated in an efficient manner, the flow conducting unit comprises according to the invention a wall and a heat retention unit which is designed to release thermal energy for decelerating a cooling down of the wall.

The flow conducting unit can especially be the flow conducting unit which is used in the method described above. Moreover, concrete elements which are mentioned in conjunction with the method can be component parts of this flow conducting unit.

The heat retention unit advantageously comprises a phase change material. The phase change material can be designed to undergo a phase change from a first phase into a second phase for decelerating a cooling down of the wall. Furthermore, the phase change material can be designed to release thermal energy during the phase change.

Alternatively or additionally, the heat retention unit can comprise a heating device. The heating device can be designed to release thermal energy, especially to the wall. The heating device can be used for example to hold the wall at, or above, a predetermined temperature threshold value, especially at, or above, 400° C.

The flow conducting unit advantageously comprises a thermal insulation. Furthermore, it is proposed that the flow conducting device comprises a heating device, especially an electric heating device. The heat device can for example be the previously mentioned heating device. The heating device can also be arranged for example between the wall and the thermal insulation. The thermal insulation expediently encompasses the wall. Furthermore, the thermal insulation advantageously encompasses the phase change material. Furthermore, the thermal insulation can encompass the heating device.

The heat retention unit expediently comprises at least one cavity. This cavity can for example be arranged between the wall and the thermal insulation. The phase change material is advantageously positioned in the cavity.

In an advantageous embodiment of the invention, the flow conducting unit has a further wall in addition to the first-mentioned wall. The further wall expediently encompasses the first-mentioned wall. The flow conducting unit can therefore be of double-walled design. The first-mentioned wall can be an inner wall and the further wall can be an outer wall. The heat retention unit can also comprise at least one cavity which is arranged between the first-mentioned wall and the further wall. The cavity can for example be delimited by the first-mentioned wall and the further wall. The phase change material is expediently positioned in the cavity. The phase change material can be seen as being macro-encapsulated.

That wall which delimits an interior space of the flow conducting unit can be understood as being the inner wall. That space through which the heat transfer medium is conducted can be understood as being the interior space of the flow conducting unit. For logical reasons, the interior space is laterally delimited by the first-mentioned wall.

In a further advantageous embodiment of the invention, it is provided that the heat retention unit has a multiplicity of cavities. The multiplicity of cavities can for example be arranged in a sponge-like structure, especially in a porous metal or in a carbon sponge. Furthermore, the phase change material can be positioned in the cavities of the sponge-like structure. The sponge-like structure together with the phase change material can be seen as being a composite material. Alternatively or additionally, the heat retention unit can comprise a dispersion of metal micro-particles, carbon particles and/or ceramic particles with the phase change material.

In a further advantageous embodiment of the invention, it is proposed that the heat retention unit comprises a plurality of capsules. Furthermore, the phase change material can be positioned in the capsules, especially enclosed in the capsules. The capsules can be understood as being phase-change bulk material. Moreover, the capsules can for example be arranged in an interior space of the flow conducting unit. The interior space can especially be the previously mentioned interior space.

The phase change material advantageously comprises a metal alloy and/or a salt mixture. For example, such a metal alloy can comprises aluminum, magnesium and zinc. Such a salt mixture can comprise for example sodium chloride, magnesium chloride, sodium fluoride, potassium fluoride and/or lithium fluoride.

The phase change temperature of the phase change material is advantageously higher than 300° C., especially higher than 400° C. Furthermore, the phase change temperature of the phase change material is expediently lower than 800° C., especially lower than 600° C.

That temperature at which the phase change takes place can be understood as being the phase change temperature. The phase change can advantageously take place if the temperature of the phase change material has a predetermined temperature value and/or if the temperature of the phase change material lies within a predetermined temperature interval. The predetermined temperature value or the predetermined temperature interval can be dependent of the chemical composition of the phase change material. During a phase change from a liquid phase into a solid phase the phase change temperature for logical reasons is the solidification temperature.

In an advantageous development of the invention, the wall has a plurality of ribs. The wall advantageously has a plurality of ribs on its side facing the heat retention unit. The ribs can have the aim of improving a heat transfer from the heat retention unit, especially from the phase change material, into the wall and/or from the wall into the heat retention unit, especially into the phase change material. The heat transfer is especially improved as a result of a larger contact surface between the wall and the heat retention unit, wherein the larger contact surface can be attributed to the ribs.

Furthermore, the wall can have a plurality of ribs on its side facing away from the heat retention unit. The side facing away from the heat retention unit for logical reasons faces the interior space of the flow conducting unit. The last-mentioned ribs can have the aim of improving the heat transfer from the heat transfer medium into the wall and/or from the wall into the heat transfer medium. In particular, the heat transfer is improved as a result of a larger contact surface between the wall and the heat transfer medium, wherein the larger contact surface can be attributed to the ribs.

Furthermore, the flow conducting unit according to the invention, especially one of its developments described above, can be an element of a power plant, especially of a thermal power plant.

The flow conducting unit is advantageously a pipeline, a steam header, a steam turbine or a heat exchanger, especially a heat exchanger of a steam generator. The power plant advantageously comprises a plurality of flow conducting units of the type described above. One of the flow conducting units can be a pipeline. Another of the flow conducting units can be a steam header. Yet another of the flow conducting units can be a heat exchanger. Another of the flow conducting units can be a steam turbine.

The previously provided description of advantageous embodiments of the invention contain many features which are reproduced in the individual dependent claims in a partially combined manner to form several others. These features, however, can expediently also be considered individually or be grouped to form practical additional combinations. These features can especially be combined individually in each case and in any suitable combination with the method according to the invention and with the flow conducting unit according to the invention. Therefore, method features, concretely formulated, are also to be seen as being characteristics of the corresponding device unit, and vice versa.

Even if certain terms are used in the description or in the patent claims in the singular in each case or in conjunction with a numeral, the scope of the invention is not to be limited to the singular or to the respective numeral for these terms. Moreover, the indefinite article ‘a’ or ‘an’ should also not impose any restriction to the singular.

The above-described characteristics, features and advantages of this invention and also the way in which these are achieved become more clearly and distinctly comprehensible in conjunction with the following description of the exemplary embodiments which are explained in more detail in conjunction with the drawings. The exemplary embodiments serve for the explanation of the invention and do not limit the invention to the combinations of features disclosed therein, nor in regard to functional features. Furthermore, for this, suitable features of each one of the exemplary embodiments, considered in an explicitly isolated manner, can also be removed from one exemplary embodiment, be introduced into another exemplary embodiment for its supplementation and be combined with any other of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing:

FIG. 1 shows a power plant with a plurality of flow conducting units;

FIG. 2 shows a cross section of one of the flow conducting units which comprises a phase change material arranged between a wall and a thermal insulation of the flow conducting unit;

FIG. 3 shows a longitudinal section of the flow conducting unit from FIG. 2;

FIG. 4 shows a longitudinal section of an alternative flow conducting unit which comprises an outer and an inner wall and also a phase change material arranged between the outer and the inner wall; and

FIG. 5 shows a longitudinal section of a further alternative flow conducting unit, in the interior space of which capsules with a phase change material are positioned.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 schematically shows a thermal power plant 2 which is constructed as a gas and steam power plant. The power plant 2 has a steam generator 4, a steam header 6 and a steam turbine 8. Furthermore, the power plant 2 has a pipeline 10 which interconnects the steam generator 4 and the steam turbine 8. The steam generator 4 comprises a heat transfer medium 12. The power plant 2 also comprises a fuel preheater 14 with a fuel feed line. The steam header 6, the steam turbine 8, the pipeline 10, the heat transfer medium 12 and the fuel preheater 14 are flow conducting units 16 of the power plant 2.

The power plant 2 also comprises a generator 18 which is connected to the steam turbine 8 via a shaft 20. The shaft has a clutch 22 between the generator 18 and the steam turbine 8. Moreover, a gas turbine 24 is rigidly connected to the generator 18 via the shaft 20. The fuel preheater 14 is arranged on the inlet side of the gas turbine 24. A combustion chamber 26 is also arranged between the gas turbine 24 and the fuel preheater 14. The gas turbine 24 is also connected on the outlet side to the steam generator 4 via an exhaust gas line 28. The power plant 2 furthermore comprises a condenser 30 and a return line 32 which connects the condenser 30 to the steam generator 4 and has a condensate pump.

During an operation of the power plant 2, a fuel, especially natural gas, is introduced into the fuel preheater 14 via a fuel feed line and preheated in said fuel preheater 14. The preheated fuel is then combusted in the combustion chamber 26 and hot pressurized exhaust gases are fed to the gas turbine 24 and expanded there, performing mechanical work. The still hot exhaust gases are then fed through the exhaust gas line 28 to the steam generator 4. The exhaust gases also flow through the steam generator 4 until they are finally released into the atmosphere through an exhaust gas outlet 34 of the steam generator 4. On their path through the steam generator 4, the exhaust gases in the heat transfer medium 12 of the steam generator 4 release their heat to process water. By absorbing the heat, the process water evaporates, wherein the steam which is produced in the process forms a heat transfer medium which is heated by the exhaust gases.

The heat transfer medium (i.e. the steam) is collected in the steam header 6 which is connected downstream to the steam generator 4. Via the pipeline 10, the heat transfer medium is directed into the steam turbine 8.

The heat transfer medium which is conducted through the steam turbine 8 drives the steam turbine 8 and expands in the process, performing mechanical work. Via the shaft 20, the gas turbine 24 and the steam turbine 8 (with the clutch 22 engaged) drive the generator 18 which generates electric energy.

The expanded heat transfer medium is condensed in the condenser 30 which is connected downstream to the steam turbine 8 and the (liquid) process water which is produced in the process is fed via the return line 32 to the steam generator 4. The process water can be reheated and the process described above is repeated in a cycle.

If the power plant 2 is run down, then a fuel feed into the combustion chamber 26 is prevented. The gas turbine 24 no longer delivers hot exhaust gases which transfer heat to the process water and to the heat transfer medium, as a result of which the heat transfer medium cools down. On account of the cooling down, said flow conducting units 16 (i.e. the steam header 6, the steam turbine 8, the pipeline 10, the heat transfer medium 12 and the fuel preheater 14) also cool down.

The flow conducting units 16 comprise in each case a heat retention unit 36 with a phase change material 38 (cf. FIG. 2 et seq.) which during operation of the power plant 2 exists in liquid form. If a temperature of the phase change material 38 falls short of a predetermined temperature during the cooling down of the heat transfer medium, the phase change material 38 solidifies. In the present exemplary embodiment, the predetermined temperature lies at 500° C. When solidifying, the phase change material 38 releases thermal energy to a wall 40 (cf. FIG. 2 et seq.) of the respective flow conducting unit 16. Moreover, when solidifying, thermal energy is released to the heat transfer medium which is contained in the respective flow conducting unit 16. Consequently, further cooling down of the wall 40 and of the heat transfer medium is decelerated.

During restarting of the power plant 2, especially within 24 to 48 hours after running down, the flow conducting units 16 have a considerably higher temperature than a flow conducting element without phase change material so that a heating up phase, within which a temperature threshold value is achieved, turns out to be significantly shorter. As a result, the power plant 2 can be operated in a more economically efficient manner. Moreover, as a result of a smaller temperature fluctuation caused by a lower cooling down thermomechanical loads upon the flow conducting units 16 are reduced so that the power plant 2 is operated in a more material-conserving manner, which is also of benefit to the profitability and efficiency of the power plant 2.

FIG. 2 shows by way of example a simplified cross section of one of the previously mentioned flow conducting units 16.

The depicted flow conducting unit 16 comprises a wall 40, a thermal insulation 42 which encompasses the wall 40, an interior space 44 through which the heat transfer medium is conducted, and a heat retention unit 36. The heat retention unit 36 in turn comprises a cavity 46 which is arranged between the wall 40 and the thermal insulation 42 and contains a phase change material 38.

Furthermore, the heat retention unit 36 comprises an electric heating device 48 which is arranged between the phase change material 38 and the thermal insulation 42.

As a result of the previously mentioned cooling down of the heat transfer medium (during the running down of the power plant 2), the phase change material 38 undergoes a phase change, also referred to as a phase transition, from a liquid phase into a solid phase, wherein the phase change takes place within the range of a solidification temperature of the phase change material 38. During the phase change (solidification), the phase change material 38 releases thermal energy, especially in the form of enthalpy of fusion, to the wall 40, as a result of which a cooling down of the wall 40 is decelerated. The wall in turn transmits the thermal energy to the heat transfer medium. Consequently, a cooling down of the heat transfer medium is decelerated in turn.

During the (re)starting of the power plant 2, the phase change material 38 is brought from the solid phase into the liquid phase. In this case, thermal energy in the form of enthalpy of fusion is absorbed by the phase change material 38 within a narrow temperature range.

In principle, a different phase change than the phase change from the liquid phase into the solid phase, especially a phase change from a solid phase into another solid phase (solid-to-solid phase change), can be used.

The phase change material 38 can comprise for example a metal alloy or a salt mixture. For example, a phase change material 38 can be a metal alloy consisting of aluminum, magnesium and zinc, especially with aluminum proportion of 59%, a magnesium proportion of 35% and a zinc proportion of 6%. Alternatively, the phase change material 38 can be for example a salt mixture consisting of sodium chloride and magnesium chloride, especially with sodium chloride proportion of 48% and a magnesium chloride proportion of 52%. The phase change material 38 can also be another salt mixture consisting for example of sodium fluoride, potassium fluoride and lithium fluoride, especially with a sodium fluoride proportion of 12%, a potassium fluoride proportion of 59% and a lithium fluoride proportion of 29%.

If the temperature of the phase change material 38 falls short of a predetermined minimum temperature of for example 450° C., then thermal energy is fed to the phase change material 38 by means of the electric heating device 48. In this case, the phase change material 38 is heated directly by means of the heating device 48. The phase change material 38 transmits the thermal energy to the wall 40. As a result, the wall 40 is held above a predetermined temperature threshold value, especially above 400° C., so that during start-up a waiting time for heating up the flow conducting unit 16, especially the wall 40, above the temperature threshold value is curtailed.

Also shown in FIG. 2 is a sectional plane III-III in the form of a dashed line.

FIG. 3 shows a longitudinal section of the flow conducting unit 16 from FIG. 2 along the sectional plane III-III. This figure does not show any additional features. Rather, it shows the flow conducting unit 16 from another perspective for the sake of a better understanding. The subsequently depicted alternative flow conducting units 16 are illustrated from the same perspective.

FIG. 4 shows a longitudinal section of an alternatively designed flow conducting unit 16. The following descriptions of FIG. 4 and FIG. 5 are restricted in the main to the differences to the exemplary embodiment from FIG. 2, to which respectively constant features and functions are referred. In the main, constant elements are basically identified by the same designations and features which are not mentioned are assumed in the following exemplary embodiments without them being described again.

The flow conducting unit 16 shown in FIG. 4 has an inner wall 50 and an outer wall 52. In the present case, the cavity 46, in which the phase change material 38 is positioned, is arranged between the inner wall 50 and the outer wall 52.

The inner wall 50 has a plurality of ribs 54 on its side facing the phase change material 38. The ribs 54 have the aim of improving a heat transfer from the phase change material 38 to the wall 50 and/or from the wall 50 to the phase change material 38. In particular, the heat transfer is improved as a result of a larger contact surface between the phase change material 38 and the inner wall 50.

The inner wall 50 also has a plurality of ribs 54 on its side facing away from the phase change material 38. The side of the inner wall facing away from the phase change material 38 faces the interior space 44 which conducts the heat transfer medium. The last-mentioned ribs 54 also have the aim of improving a heat transfer from the heat transfer medium to the wall 50 and from the wall 50 to the heat transfer medium. In particular, the heat transfer is improved as a result of a larger contact surface between the heat transfer medium and the inner wall 50.

The ribs 54 of the inner wall 50 are optional. In principle, the ribs 54 can be omitted on the side facing the phase change material 38 and/or on the side facing away from the phase change material 38. In the case of a flow conducting unit 16 of single walled design (according to FIG. 2 and FIG. 3), the ribs 54 could also be provided on the side of the wall 40 facing the phase change material 38 and/or on the side facing away from the phase change material 38.

In the present case, the heating device 48 is arranged between the outer wall 52 and the thermal insulation 42. If a temperature of the phase change material 38 falls short of a predetermined minimum temperature, then thermal energy is fed to the phase change material 38 by means of the electric heating device 48. In this case, the phase change material is heated indirectly. The electric heating device 48 heats the outer wall 52 of the flow conducting unit 16. The thermal energy which is absorbed by the outer wall 52 in the process is transmitted to the phase change material 38 and from the latter to the inner wall 50.

FIG. 5 shows a longitudinal section of a further alternatively designed flow conducting unit 16 with a phase change material 38. In the present case, the phase change material 38 is not enclosed between the wall 40 and the thermal insulation 42 but in a plurality of capsules 56 which are positioned in the interior space 44 of the flow conducting unit 16. In other words, the heat retention unit 36 in this case comprises the capsules 56 with the phase change material 38 in addition to the heating device 48.

The phase change material 38 releases thermal energy to the heat transfer medium during the phase change, especially during the solidification. The heat transfer medium in turn transmits the thermal energy to the wall 40.

If a temperature of the phase change material 38 falls short of a predetermined minimum temperature, then thermal energy is fed to the phase change material 38 by means of the electric heating device 48. In this case, the phase change material is heated indirectly. The heating device 48 heats the wall 40 of the flow conducting unit 16. The thermal energy which is absorbed by the wall 40 in the process is transmitted to the heat transfer medium. The heat transfer medium in turn transmits the thermal energy to the capsules 56 and to the phase change material 38 which is enclosed in the capsules 56.

In the case of the previously described exemplary embodiments, the phase change material can in principle even be dispensed with. In such a case, the heating device can release thermal energy directly to the wall and as a result the cooling down of the wall can decelerate.

Although the invention has been fully illustrated and described in detail by means of the preferred exemplary embodiments, the invention is not limited by the disclosed examples and other variations can be derived therefrom by the person skilled in the art without departing from the extent of protection of the invention. 

1. A method for decelerating a cooling down of a flow conducting unit, the method comprising: exposing the flow conducting unit which comprises a heat retention unit to throughflow by a heat transfer medium, reducing a temperature of the heat transfer medium, as a result of which a wall of the flow conducting unit cools down, and the heat retention unit releases thermal energy, as a result of which a cooling down of the wall is decelerated.
 2. The method as claimed in claim 1, wherein the heat retention unit comprises a heating device which releases thermal energy.
 3. The method as claimed in claim 2, wherein a temperature of the wall is held at, or above, a predetermined temperature threshold value by the heating device.
 4. The method as claimed in claim 1, wherein the heat retention unit comprises a phase change material, wherein the phase change material undergoes a phase change from a first phase into a second phase, and wherein the phase change material releases thermal energy during the phase change, as a result of which the cooling down of the wall is decelerated.
 5. The method as claimed in claim 4, wherein the thermal energy released by the phase change material during the phase change is released to the wall which transmits the thermal energy to the heat transfer medium, and/or in that the thermal energy released by the phase change material during the phase change is released to the heat transfer medium which transmits the thermal energy to the wall.
 6. The method as claimed in claim 4, wherein thermal energy is fed to the phase change material by a heating device.
 7. A method for operating a power plant, comprising: decelerating a cooling down of a flow conducting unit of the power plant as claimed in claim 1, wherein the temperature of the heat transfer medium which flows through the flow conducting unit is reduced as a result of a reduction of an output of the power plant.
 8. A flow conducting unit, comprising a wall, and a heat retention unit which is designed to release thermal energy for decelerating a cooling down of the wall.
 9. The flow conducting unit as claimed in claim 8, wherein the heat retention unit comprises a phase change material which is designed to undergo a phase change from a first phase into a second phase for decelerating a cooling down of the wall and to release thermal energy during the phase change.
 10. The flow conducting unit as claimed in claim 9, further comprising: a thermal insulation and a heating device, wherein the heating device is arranged between the wall and the thermal insulation and the thermal insulation encompasses the wall, the phase change material and the heating device.
 11. The flow conducting unit as claimed in claim 9, further comprising: a further wall which encompasses the first-mentioned wall, wherein the heat retention unit comprises at least one cavity which is arranged between the further wall and the first-mentioned wall and in which is positioned the phase change material.
 12. The flow conducting unit as claimed in claim 9, wherein the heat retention unit comprises a plurality of capsules in which the phase change material is positioned and which are arranged in an interior space of the flow conducting unit.
 13. The flow conducting unit as claimed in claim 9, wherein the phase change material comprises a metal alloy and/or a salt mixture and a phase change temperature of the phase change material is higher than 300° C., especially higher than 400° C., and lower than 800° C.
 14. The flow conducting unit as claimed in claim 8, wherein the wall has a plurality of ribs on its side facing the heat retention unit and/or on its side facing away from the heat retention unit.
 15. A power plant comprising: a flow conducting unit, as claimed in claim 8, wherein the flow conducting unit is a pipeline, a steam header, a steam turbine or a heat transfer medium.
 16. The method as claimed in claim 2, wherein the heating device releases thermal energy to the wall.
 17. The method as claimed in claim 6, wherein thermal energy is fed to the phase change material by the heating device if a temperature of the phase change material falls short of a predetermined minimum temperature.
 18. The flow conducting unit as claimed in claim 10, wherein the heating device is an electric heating device.
 19. The flow conducting unit as claimed in claim 13, wherein the phase change temperature of the phase change material is higher than 400° C., and lower than 600° C.
 20. The power plant as claimed in claim 15, wherein the power plant comprises a thermal power plant. 